Telephone system



Aug. '12, 1941.

J. H.- voss 2,252,309

TELEPHONE SYSTEM Filed Feb. 7, 1938 14 Sheets-Sheet 2 @SUBSCRIBEQSI LINE CIRCUIT I SUBSCRIBERS LINE l6 CIRQU IT 22 AUTOMATIC /30 EXCHANGE TRUNK IEINE CIRCUIT RA392 RC333 L-! TO CONN.

INVENTOR. JOHN H. VOSS BYM,M7,M m Q ATTORNEY 5,

Aug. 12, 1941. H voss -2,252,309

TELEPHONE SYSTEM Filed Feb. 7, 1938 14 Sheets-Sheet 3 ATTORNEY 5.

Aug. 12, 1941. J. H. voss TELEPHONE SYSTEM Filed Feb. 7, 1938 14 Sheets-Sheet 7 N3 m v3 INVENTOR. JOHN H. VOSS mono h mhhu ATTORNEYS.

Aug. 12, 1941. J. H. VOSS TELEPHONE SYSTEM Filed Feb. 7, 1938 14 Sheets-Sheet 8 ATTORNEYS Aug. 12, 1941. 055 2,252,309 TELEPHONE SYSTEM Filed Feb. '7, 1938 14 Sheets-Sheet 9 (I) w 2E Z i- I INVENTOR. JOHN H. VOSS M W,M%ZM

ATTORNEYS.

,Aug. 12, 1941. J. H. voss TELEPHONE SYSTEM Filed Feb. 7, 1938 i4 Sheets-Sheet 1o NmU RAIOG m2 mbjwm 0595 E.

INVENITOR JOl -|N H. voss KYzww My M ATTORNEYS,

Aug. 12, 1941. J. H. voss TELEPHONE SYSTEM Filed Feb. 7, 1938 14 Sheets-Sheet l4 mmio 33o mm Eu RNEQ w m E vwiu mwSu

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IIIHIIH L 5 moi 8 2 M Em jooim EE mac INVENTOR. JOHN H. VOSS BY m M ATTORNEY-5.

' ment including in a definite order.

Patented Au 12,, 1941 UNITED! STATES,"

rater-noun SYSTEM John B. Voss, Downers Grove, -Ill. asslgnor Associated Electric Laboratories, Inc., 1 7.

11]., a corporation of De laware Chicago,

Application Feb uary 7, Sofia! 1 2 I x I. Q 104 Claims. (cl. 179-47). I

The present invention relates to telephone systems and has for one of its objects the provision of improvements in small automatic systems of the type adapted for installation in small towns or villages.

An automatic telephone system of the character noted is generally known as a community automatic exchange or C. A. X. and usually has a line capacity of one hundred lines or less. Certain of the subscribers lines may be of the metallic, two-conductor variety while others of the subscribers lines may be of the well-known ground return type. The metallic lines are usually of the multi-party type and bridged or divided code ringing is utilized for selectively signaling the substations thereon. In order to provide toll service to the subscribers, provisions are also made for trunking calls to and from one or more distant exchanges. One improved allrelay exchange of this type is disclosed and claimed in the copending application, Serial No. 188,443, Frank Kessler, filed Feb. 3, 1938, and the present invention is directed more particularly to further improvements in the circuits illustrated and described in this copending application. In brief,'in the system as disclosed in the Kessler application, the subscribers lines and trunk lines of the system extend to the finder and connector switches of each of a plurality of all-relay finder-connector links, common equipa relay type link distributor or allotter being provided for assigning idle ones of the links to successive calling ones of the lines various links are fully directive in character; that is, all lines hunting operations are eliminated through an arrangement whereby only the group and unit relays corresponding to a particular calling line are caused to operate when a link is taken for use. 'For the purpose of decreasing the cost of the links, the number of unit relays provided in the finder and connector portions of each linkare reduced by half through the provisions of a subgroup relay. By providing such subgroup relays, each unit relay of each line selecting finderor connector switch is rendered operative to select either of two lines depending upon the operated or non-operated condition of the associated subgroup relay. In the particular disclosed arrangement of the finder switches,

however, there is a possibility of a busy line being momentarily connected through to the link circuit of a seized link, to the annoyance of the conversing parties, due to the fact that no apparatus is provided for insuring the operation of The finder switches of the 5 no provisions are made for fully utilizing the unit relays of the various connector switches to accommodate the selection of trunks arranged in groups of more than five lines, these relays only being individually operative to select for use a m single corresponding trunk when the corresponding trunk is idle. Also, no distinction is made between the trunk lines and the subscribers lines as regards the method of arranging the line circuits individually associated therewith. Provisions are made for the selection of coded ringing current for signaling desired substations on the multi-party lines, the code selection being entirely performed in any operating link through the response of the link circuit apparatus to a code selecting digit which follows the line selecting digits required for the selection of the desired called line.

It is a further object of the present invention to provide for use in a system of the character 2.3 noted an automatic line selecting switch of the all-relay type which is fully directive in character, which requires a minimum number of relays both in the switch itself and in the associated common equipment, and which is operativeto preclude the connection of more than one line to its associated link circuit during a line selecting operation.

It is another object of the invention to provide an improved arrangement of the allotter or dis- 5 tributor equipment associated with the all-relay finder-connector links, whereby a new link is substantially instantaneously assigned to the use of a calling line in fails to find a calling line, and which functions 40 in a manner such that there is no possibility of more than, one link seizing the same calling line at the same time or more than one calling line being seized by one link. I

It is a further object of. the invention to .pro-

vide an improved line terminating circuit which is so arranged that the line associated therewith is immediately marked as busy in the connector switches to which it extends when a calling condition is created on the line and before the finder switch of anassigned link functions to seize the line.

It is an additional objectof the invention to provide an improved finder-selector link circuit arrangement whereby either of two different line circuit arrangements may be employed in termithe event a first assigned link nating different ones of the lines extending to the link.

It is a further object of the invention to provide in a link of the character described an improved arrangement for determining the type of ringing to be utilized in ringing over a selected called line and for determining the code of the ringing current to be projected over the selected called line.

It is a further object 01 the invention to provide, in a link circuit arranged to provide divided ringing on multi-party metallic lines, an arrangement for preventing the ringing current source from being short-circuited when ringing over a selected line of the ground return type.

It is another object of the invention to provide improved apparatus for registering the operation of a link each time the link is involved in a connection.

It is a still further object of the invention to provide an improved arrangement for selecting idle ones of a group of trunk lines accessible to a link, which arrangement is such that the number of selecting relays utilized in each of the several links is reduced to a minimum.

The features of the present invention are illustrated as being incorporated in an exchange having an ultimate capacity of one hundred lines, ten of which are two-way trunk lines extending to a distant exchange and the remaining ninety of which are subscribers lines. The various lines individually terminate in line circuits which are differently connected and arranged depending upon whether the terminated line is a trunk line or a subscriber's line. In accordance with one feature of the invention, the control apparatus of any assigned finder-connector link is so arranged that the line circuit associated with any calling one of the lines is caused to operate when the assigned link finds the calling line irrespective of which type of line is responsible for the seizure of the link. More specifically, each line circuit subscriber's line is of the usual type wherein ousying potential is only applied to' the private or control conductorof the line following the operation of an assigned link to seize the associated line when a calling condition is created thereon. In each of the line circuits terminating a trunk line, an improved arrangement is utilized which insures that the terminated line will be marked as busy immediately a calling condition is created thereon. To this end, each of the last-mentioned line circuits has embodied therein a line relay which operates to apply a predetermined busying potential to the private or control conductor of the terminated'line when a call is initiated on the line. Also, each of the lastmentioned line circuits is provided with a cutoff relay which is operative to disconnect the associated line relay from across the associated line only in response to the application of a predetermined potential to one of the talking conductors of the associated line. In order to make each link circuit universally effective to control either form of line circuit, there is provided therein control apparatus for applying busying ground potential to the control conductor of a seized calling line and also to the line conductor of the link line by way of which the cut-off relays of the trunk terminating line circuits are.

energized.

For the purpose of selecting idle ones of the links for the use of calling ones of the lines and for the further purpose of controlling the operawhich terminates a tion of the finder switch'of any assigned link to seize the calling line to which it is assigned, there is provided in association with the several links common equipment including a group of allotter relays individually corresponding to the links and a group of auxiliary relays individually corresponding to the unit relays of the respective finder switches. The allotter relays are connected and arranged sequentially to operate to select idle ones of the links and are individually provided with are so arranged that the locking circuit for each relay is opened in response to the operation oi the next succeeding relay. The allotter relays are arranged to control start circuits individually extending to the various links. These start circuits are successively prepared during the sequential operation 01 the allotter relays and an improved arrangement is provided for maintaining the start circuits open independently. or the allotter relays during the sequential operation of these relays. In order to prevent more than one of the allotter relays from inadvertently being locked operated in a link-assigning position at the same time, there is provided an additional relay which is operative to prevent two of the locking circuits from being held completed at the same time for a prolonged time interval. Provisions are also made in the allotter for assigning a new link to the use of a calling line in the event a first assigned link fails to find the line. To this end and in accordance with a further feature of the invention, there is provided a relay which is conditioned to operate from one position to a second position in response to the assignment of one of the links to the use of a particular calling line, and the allotter relays are arranged to respond to the operation of this relay to its second position to assign a second link to the use of the calling line. This relay is constructed to have an operating characteristic such that the time required for it to operate from its first to its second position, following the conditioning thereof for such operation, exceeds the time normally required for the finder switch of one of the links to seize a calling line. The circuit arrangement is such that if the first assigned link is successful in finding the calling line the relay is prevented from operating to its second position.

The arrangement of the unit relays provided in each oi the finder and connector switches is similar to that employed in the system 01 the above-referred-to copending application in that the lines of each line group are divided into subgroups and each unit relay'is operative to select either of two different lines individually included in different subgroups depending upon the operated or non-operated condition of an associated subgroup relay. In accordance with a further feature of the present invention, however, the operation of any unit relay provided in any one of the finder switches and corresponding to a line to be selected is made dependent upon the prior operation of the associated subgroup relay I in any case where the selection of the line requires that both the subgroup relay and the unit relay, corresponding to the line, be operated.

. More particularly, each finder switch is arranged so that the subgroup relay provided therein operates when a calling line is included in one subgroup of lines and, upon operating, completes the operating circuit for the one unit relay which is operative to select the calling line, whereas this oneunit relay is caused to operate looking or holding circuits which sired called subscribers line,

independently of the. subgroup relay when it is included in the other subgroup of lines. By this arrangement there is eliminated any possibility of abusy line momentarily being connected through to the link circuit of an assigned link.

In accordance with a still further feature of the invention the connector switch of each link is provided with a subgroup relay which is connected and arranged to cdoperate with the unit relays of the switch in selecting idl ones of the group of ten trunks terminating at the switch. More particuarly, in association with the trunk lines, which relays are so arranged that an idle trunk line is selected immediately a previously selected trunk line is made busy. The function of any one of the connector switches on a trunk call is, therefore, to

previously. been selected by the two sets of relays noted. To this apparatus provided in each link is arranged to cause the operation of the one of the unit relays which is operative selected idle trunk, without causing the operation of the subgroup relay, so long as certain of the trunk lines are idle, and is arranged to cause the operation of both the subgroup relay and one of the unit relays when more than a predetermined number of the trunk lines are busy.

Each link is provided with a group of impulse counting relays which function to control the connector group and unit relays during the operation of the connector portion of the link to select a desired called line. Two successive operations are required on the counting relays to cause the selection of a dethe first line selecting operation of the counting relays being effective to cause the selection and operation of the group relay in the connector switch which corresponds to the particular group of ten lines including the called line, and the second operation being utilized to cause the selection and operation of the particular unit relay called line, or the operation of this relay and its associated subgroup relay. For the purpose of preventing the ringing current source from being short-circuited in ringing over a subscribers line of the ground return type, improved apparatus is provided for preventing the ground conductor of a line of this type from being connected to the ungrounded output terminal of the source during the ringing operation. At least a portion of the subscribers lines are of the multi-party type and certain thereof are arranged for divided code ringing. Others of the multi-party lines are arranged for bridged code ringing. This means that each link must be equipped to provide code ringing of either type and that suitable provisions be made for selecting the type of ringing as well as the particular code of the ringing current to b projected over the called line. In accordance with another feature of the invention, apparatus is provided in each link which retwo sets of relays are provided corresponding to the part of the impulse sponds to the operation of the assoicated impulse counting relays during the dialing ceding the line selecting digits whether bridged or divided ringing in ringing over a selected one of the subscribers lines. This apparatus also functions at least partially to select the code of the ringing current to be projected over the selected line. More specifically,

of a digit prethe operation of this apparatus is dependent upon the number of impulses comprising the first digit dialed into the counting relays from a calling substation and, if the digit comto determine. will be utilized prises ten impulses, it does" not operate. In the case of a ten-impulse first digit, certain of the control apparatus embodied in the link circuit Further features of the invention pertain to the particular arrangement of the circuit elements whereby the above-outlined and additional operating features are attained.

The novel features believed to be characteristic of the invention are set forth in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages therof, will best be understood by reference to the specification taken in connection with the accompanying drawings illustrates the general arrangebriefiy outlined above.

Referring now more particularly to Fig. 1 of the drawings, the system there shown comprises a small community exchange l0. and a second automatic exchange 30 interconnected by a group of two-way trunk lines, one of which is indicated in the drawings at M50 as terminating at a repeater circuit 20 forming a part of the apparatus located at the exchange "I. Also extending to the exchange In are a plurality of subscribers lines, three of which are indicated at ll, l2 and I6. Each of the enumerated subscribers lines is illustrated as being of the multi-party type having two or more telephone substations associated B and C are shown in association with the line l6 and two substations D and E are illustrated as being connected to the line l2. Each of the substations connected to the lines may comprise the usual hand set telephone having embodied therein a dial controlled impulsing device of conventional arrangement. Each of the lines extending to the exchange l0 terminates in a line circuit through which it is rendered accessible to the finder and connector switches included in the exchange. Thus, the

line H terminates at a line circuit 2| through which it is connected to the contact springs of a finder switch 24 and a connector switch 26, which two switches are connected together .to form a finder-connector link., Similarly, the line I6 terminates at a line circuit 22 and the illustrated two-way trunk The exchange III is designed to serve a relatively small number of subscribers lines, specifivarious subscribers line circuit contemplated that certain cally 60, and the trunk lines of a ten-line trunk group and, accordingly, all of the subscribers lines and trunk lines are arranged to be served by any one of the plurality of finder-connector links included therein. Obviously, the number or links required for satisfactory service will depend upon the volume of telephone trafiic but, taking into account the character of the exchange, ten links are considered adequate for reasonably good service. In order to assign the links to successive calling lines in a definite order so that a link is always available for the use of a calling line, excepting, of course, those instances when all of the links are simultaneously busy, there is provided certain common equipment 25 which includes a link distributor or allotter. The several links also have commonly associated therewith ringing, tone and timing apparatus which is diagrammatically shown at 21,

Neglecting for the present the details of the circuit arrangement, and considering briefly the operation of the system, when a subscriber, such, for example, as the subscriber at the telephone A, initiates a call over the line II to a second subscriber's telephone, such, for example, as the telephone C, by lifting his receiver from its hook, the

2| operates to cause the finder-connector link previously assigned by the distributor to the calling line, to be started in operation to seize the calling line. Assuming that the link 24, 26 is delegated by the distributor to set up the connection, the finder portion 24 erates to select the calling line and to connect it through to the conductors of the link line incoming to the connector portion 26 of the link. When the calling line ii is seized and extended by way of the finder switch to the associated connector switch, it is marked as busy in all of the other connector switches and the control equipment of the seized link is automatically conditioned to respond to the impulses of the first series of impulses transmitted from the impulsing device at the calling substation A. Since the subscribers lines are of the multi-party type, provision must necessarily be made for selectively signaling the desired called substation on any selected line. With the arrangement to be described more in detail hereinafter, coded ringing is utilized to accomplish this end. It is further of the lines may be arranged for bridged ri ging and others of the lines may be arranged for divided ringing and, in accordance with one feature of the present invention, the link circuit apparatus of any seized link arranged to respond to the impulses of the first digit ,dailed at lect the type of ringing to be used and further, if bridged ringing is selected, partially to select the code of, the ringing current subsequently to be projected over the called line. Further in accordance with this feature of the invention, the operation of the link circuit apparatus during the first digit determines whether the selected line will be a called subscriber's line or a trunk line extending to the distant exchange. Also, if the number of impulses making up this first digit is more than a predetermined number, a registering device provided in the link is caused to operate to register the operation of the link.

.In the case under consideration wherein the call is to be extended to the subscribers line i6, the connector switch 26 will respond to the impulses of the first digit to select the type of ring-,

of the link opthe link to be registered or recorded. If the called line l6 happens to be one of the lines arranged for bridged ringing, the first digit also results in the partial selection of the ringing code to be used in ringing over the called line.

The connector switch 26 responds to the impulses of the second digit dialed at the callin substation by selecting the group of ten lines which includes the called line it. Thereafter. and in response to the dialing of the third digit, the called line i6 is selected from the selected group of lines. When the called line is selected, the control apparatus included in the link circuit functions to test this line for the purpose of determining the busy or idle condition thereof.

Busy tone current, as generated by a portion of the common apparatus 21, is not returned over the loop extending to the calling subscriber's substation until the fourth or code selecting digit is dialed at this substation. When this digit is dialed, the link circuit apparatus functions to select the particular code of the ringing current to be projected over the called line for signaling the substation desired by the calling party. At the conclusion of this digit and if the called line is busy, busy tone current is returned over the loop to the calling subscriber's substation to indicate to the calling party that the desired connection can not immediately be completed. If, on the other hand, the called line is idle at the time it is selected, ringing current of the selected code corresponding to the called substation C is projected'over the called line for the purpose of energizing the signaling devices individually provided in association with at least a portion of the substations on the called line. In this connection it will be understood that, if divided ringing is utilized, the ringing current is projected over the side of the line to which the ringing device at the desired substation is connected. On the other hand, if bridged ringing is selected as the type of ringing to be used, ringing current is conducted over both sides of the called line to energize all of the ringing devices individually provided at the substations connected to the line. During the ringing operation, ring-back tone current is applied to the loop extending to the calling substation to indicate to the calling party that the desired substation is being signaled. Finally, a connection between the calling and called substations is completed when the call is answered at the substation C. As pointed out the calling substation to seing to be utilized and to cause the operation of station.

with particularity hereinafter, the arrangement of the link circuit apparatus is such that a link utilized in settingup a desired connection is released only when the connection is cleared out at the calling substation.

The switching apparatus located in the exchange ill may also be utilized in setting up a connection between two substations connected to the same line, the sole function of the apparatus in this regard being that of selecting and applying ringing current to the calling line of the proper code to signal a party at the called sub- The discharge of this function necessitates the seizure of the calling line by an idle link, all in the manner outlined above. Thereafter, and following the dialing of the four digits of the called substation directory number at the calling substation, the calling party restores his receiver to its hook. When the calling party hangs up, the finder portion of the operated link is released, and ringing current of the selected code identifying the called substation is projected over the calling line. ,If the calling line is one of the lines arranged for divided ringing, the link circuit apparatus functions to project areverting call signal ringing current over the line conduccircuit of the line in service operates to busy the line, thereby to prevent its seizure by the connector portion of another link on a second call to.

one of the substations on the line. Finally, the line circuit of the line in use is restored to normal when the connection is cleared out at the calling and called substations.

In the operation of the link to select one of the group of trunk lines interconnecting the exchange I. and the exchange 30, only one digit comprising a predetermined number of impulses is dialed at the calling substation. The connector portion of the seized link responds to the impulses of this one digit to select a preselected idle one of the group of trunk lines. More specifically, these lines have associated therewith trunk preselecting apparatus which functions to select an idle line for use immediately a previously selected line is taken for use. Hence, at the conclusion of a trunk selecting digit dialed at a calling substation, the connector portion of the seized link immediately operates to. seize the preselected idle trunk.

.When the trunk line is seized, the calling line is switched through by way of the selected trunk line and its associated repeater circuit to the terminating apparatus located in the distant exchange. In this connection, it is noted that two sets of link line conductors are provided in the link circuit of each link, one of which is. devoid of all voice frequency coupling elements, such, for example, as condensers or repeating coils, and is utilized in extending a desired connection by way of one of the trunk lines. sion of the connection to the trunk line terminating apparatus located in the distant exchange 30, the connection may further be extended to the desired called line through the dialing of the regular directory number corresponding to the desired line at the calling substation. The manner in which the connection is released following the establishment of a connection to the desired line depends upon the nature of the repeater circuit associated with the trunk line which is used in setting up the connection.

Since the trunk lines interconnecting the exchanges l and 30 are of the two-way type, it will be understood that any one of these lines and the switching apparatus included in the exchange l0 may be utilized in the completion of calls originating or extended by way of the exchange 30 and which are to be extended to one of the subscribers lines terminating at the exchange I 0. The mode of operation of the equipment in the exchange l0 following the extension of a call thereto by way of one of the trunk lines noted is substantially similar to that set forth above with reference to the connection established between the substations A and C, it being observed that each of these trunk lines terminates in a line circuit connected and arranged in a novel manner which is different from the arrangement of the line circuits individually terminating the subscribers? lines. In this case, the release of the connection is entirely Following the ext'en-,

' referring. to Figs.

under the control of the calling party whose'line terminates at the distant automatic exchange.

Considering now in greater detail the apparatus embodied in the exchange l0 and involved in the above-described sequence of operations, and

3 to 14, inclusive, there are illustrated the elements of the line circuits respectively terminating the finder-connector link 2.4, 26, and of the comline circuits individually terminating the subscribers lines are of more orless conventional arrangement. More particularly, the

line circuit 2| comprises a line relay R300, a cut-off relay R3I0, and a line lockout relay R320 connected and arranged to operate in the usual manner. The corresponding relays for the line circuit 22 are indicated at R300, R3l0' and R320. This last-mentioned circuit is indicated as having associated therewith a pay station tone source 338 which is connected through a condenser 36'! to impress a pay station tone current on the control or private conductor C306 of the line l6 for the purpose of indicating to a supervising operator in a distant exchange as, for example, the exchange 30', that the substation B is of the pay station type. This pay station tone circuit is shown in association with the line It for illustrative purposes only, it being pointed out that a circuit of this form is provided only in conjunction with those of the lines terminating at the exchange l0 which are of the single party pay station type. Since the apparatus for applying pay station tone current to the tional in character, it

control conductor C306 may be entirely convenhas been only diagrammatically illustrated.

The line circuits individually terminating the respective trunk lines are identical in character and are differently connected and arranged from the line circuits terminating the subscribers lines.

In brief, the line circuit 23 comprises a line relay R350, a cut-oif relay R330, a lockout relay R340 and a switching relay R330. The primary difference between this line current and the subscriberfs line circuit 2|, for example, pertains to the method of busying the associated lines to calls incoming thereto. With this illustrated circuit arrangement of the line circuit 23, the line relay R350 is operative to impress a line busying poten- 'tial on the private or control conductor C353 immediately a calling condition is created on the associated line, and the cut-oil relay R330 is operative over a circuit including the positive talking conductor C351 of the line. With the subscribers line circuit 2|, on the other hand, a line busying potential is only applied to the control conductor C306 following the operation of one of the finder switches to seize the line, and the cutoff relay R3! 0 is only operative when the busying potential is applied to the control conductor C306.

Each of the finder switches is of the fully directive all-relay type, group and unit relays being provided for performing the line selecting functions. The lines terminating thereat are arranged in groups of ten lines and the lines of each group terminate at the contact springs of one group relay. More particularly, the finder switch 24 comprises ten group relays corresponding to and including the two relays R310 and R380 individually having contact springs for terminating the conductors of ten lines. While the group relays may be of any-desired commercial type the lines ll, l6 and I450, of

a line is in one subgroup having the required number of contact springs, preferably these relays are of the improved form disclosed and claimed in the copending'application Serial No. 109,813, Herbert F. Obergfell, filed November 9, 1936. The lines of each group are divided into two equal subgroups and through the provision of a subgroup relay in each finder switch a group of five unit relays is sufiicient to permit the selection of any desired one of a selected group of ten lines. Thus, the finder switch 24 comprises a subgroup relay R430 and five unit relays corresponding to and including the two relays R400 and R4 I 0, each of which unit relays is operative to select either of two lines in a select-. ed group of'ten lines depending uponthe operated or non-operated condition of the subgroup relay R430. The finder switch 24 also in-.

' cludes a pair of switching'relays R440 and R450 which are arranged to be controlled by way of the common equipment illustrated in Figs. 5 and 6 and are operative to initiate the operation of the finder switch to seize a calling line.

The common equipment shown in Fig. 5 includes a plurality of auxiliary relays which are arranged to be energized over start circuits controlled by the line circuits associated with the respective lines terminating at the exchange [0. These relays are selectively operative in response to the selective completion of the start circuits and individually function to condition for operation a particular group relay and a particular unit relay embodied in the finder portion of an assigned link. More particularly, ten auxiliary group relays including the two relays R5l0 and R520 are provided, which individually correspond to the finder group relays of each link. A group of five auxiliary unit relays, including the two relays R530 and R540, are similarly provided which individually correspond to the unit relays provided in the respective finder switches and are operative to condition the corresponding finder switch unit relays for operation. The energization of the subgroup relay in the finder portion of any assigned link is dependent upon the operated or non-operated condition of a subgroup relay R500 provided in the common equipment, this latter relay also being arranged to be energized over those of the start circuits which are adapted to be completed in the line circuits terminating the lines included in predetermined ones of the line subgroups. Specifically, the arrangement of the apparatus is such that when a calling condition is created on one of the lines terminating at the exchange, start circuits are completed by the line relay associated with the calling line for causing the operation of the one of the auxiliary group relays R5l0, R520, etc., which corresponds to the group of lines including the calling line and for energizing the one of the auxiliary unit relays R530, R540, etc., which is operative to select the calling line. If the calling of its particular group the auxiliary subgroup relay R500 is not energized over the completed sta'rt circuits, but if the calling line is in the other subgroup, R500 is energized and operates to condition the subgroup relay embodied in the finder switch of the selected link for operation. In order to give the finder switch of any assigned link ample time to seize the calling line to which it is assigned and to connect the line through to the connector portion of the link before the auxiliary group and unit relays are reconditioned to respond to another call, the slowto-release relay R550 is provided, which relay is operative immediately a link starts to seize a R600, a link start relay R604 and six additional relays R6l0, R620, R630, R640, R650, and R660. Of these six additional relays, the 'slow-to-release relay REM and the relay R660 are provided for the purpose of opening the locking circuits for all of the allotter relays when two or more of the allotter relays are inadvertently locked operated in the link assigning position at the same time, and for automatically causing a single allotter relay corresponding to an idle link to be operated to its link assigning position following interruption of the allotter relay locking circuits. The relay R620, the slow-to-release relay R630, and the slow-to-operate relay R640 cooperate to cause the allotter relays to assign a second idle link to the use of a calling line in the event a first assigned link is found defective and fails to operate properly in response to the operation of the start relay R604. The relay R620 also functions to control the all-links busy relay R600 in a manner such that a multiple holding circuit is provided for maintaining R600 operated during the operation of an assigned link to seize the calling line, which holding circuit is opened to permit R600 to restore following the complete operation of the assigned link to seize the calling line and in the event the assigned link is the last available idle link. In addition, the relay R620 functions to hold open the link start circuitsindividually extending to the various links during the operation of the allotter relays to select an idle link. In order to insure the continued operation of R600 for a short interval after the allotter relays have operated to select an idle link, the slow-to-release relay R650 is provided.

The link circuit proper is illustrated in Figs. 7 to 10a of the drawings. The link line 41! incoming to the link circuit terminates in a, repeater 180 and the talking conductors thereof are arranged to be included in the operating circuits for two line relays R1l0 and R1l5. The relay R1l0 is the usual impulse responsive relay and in combination with the two additional relays R140 and R forms an impulse correcting network for repeating impulses transmitted thereto from a calling substation over an impulsing circuit to the slow-to-release control relay R960 and the impulse counting relays illustrated'in Figs, 10 and 108.. R160, this relay being arranged to be held energized by a slow-to-release slave relay R150. The

relay R100 is the usual switching through relay which is energized only when the link is utilized in extending a connection by way of one of the trunk lines to the distant exchange 30. The battery reversing relay is indicated at R115, this relay being arranged to operate in the usual manner when a call is answered at a called substation, and functions to control the operation of a slave relay R which actually performs the battery reversing operation. Any selected called line is electrically balanced through the provision of an impedance element 114 having an impedance substantially equal to that of the battery reversing relay R115. For the purpose of preventing the link from being permanently tied up through functions to open the operatin I ten links corresponding to and The link hold relay is indicated at 

